qt5.qtbase: fix cross compilation

This commit fixes builds of `pkgsCross.*.qt5.qtbase` by:

- Adding the buildPlatform compiler to depsBuildBuild in qtbase.nix
  and qtModule.nix.  The `qtbase` build machinery expects to find it
  in the $PATH in unprefixed form.

- Setting the `PKG_CONFIG_SYSROOT_DIR` and `PKG_CONFIG_LIBDIR`
  environment variables when compiling a cross-targeted `qmake`.
  This is required; if these environment variables are unset,
  `qmake` won't even try to use `pkg-config`.

- Adding the `-device` and `-device-option` flags necessary for
  cross compilation to `configureFlags`.

- Adding the (one-entry at the moment) Rosetta Stone for QT-5 as a
  `let`-defined `qtPlatform` function which takes a nixpkgs platform
  and returns a QT-5 `mkspecs`-string.

Co-authored-by: Christoph Neidahl <christoph.neidahl@gmail.com>

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